While I'm no expert on making animated images myself, you might be able to get a few tips from experienced users here:
Graphic Design 
As for what's allowed, as long as it's not conflicting with
the site rules, and is within the signature guidelines you should be fine.
Quote (http://forums.d2jsp.org/info.php?p=9)
1. The entire size of your signature shall not exceed 400 pixels wide by 150 pixels high.
2. Text sections of your signature shall not exceed 6 lines of text if you include no images, and 3 lines of text if you have an image.
3. Total size of the images contained in your signature shall not exceed 70 kilobytes.